Critical electronic circuitry, from a security viewpoint, is that which is susceptible to eavesdropping or otherwise may be altered or disabled without the knowledge of a subscriber or owner of a system or apparatus having such circuitry. For example, such electronic circuitry may be that used for encrypting or decrypting communications signals, or for electronic control circuitry in remote automated financial service terminals. The likelihood of illegal access to, or tampering with, any such circuitry is directly proportional to the profit which may be accrued by the tamperer, and inversely proportional to the degree of difficulty expected by the potential tamperer.
Two basic approaches have been used in the provision of security packages. One has been to provide a housing sufficiently impenetrable so that application of sufficient force to breach the wall of the housing will likely result in the contents being rendered valueless or alternatively will be prohibitively expensive or time consuming. Another approach has been to sensitise the housing by some means such that an occurrence of tampering is readily detected, whereby appropriate subsequent action may be effected.
In examples of the later approach, the wall of a housing includes one or more electrical conductors which may be monitored for continuity. In one example, the breaking of any one conductor results in a loss of continuity, which may be evidence of tampering. However in this example, the appropriate electrical conductor may be bridged with another conductor being placed by the tamperer, prior to the breakage, in order to conceal the occurrence of tampering.
In another more sophisticated example, one or more electrical conductors are arranged to be of a convenient predetermined resistance or may include segments of predetermined resistances. Tamper detection circuitry includes resistance measurement means which is adjusted at the time of manufacture to have an all seems well range. During use, if the resistance of a monitored electrical conductor changes to a value outside of the all seems well range, this is taken to be an indication of possible tampering. Unless the tamperer has acquired a very detailed knowledge of the particular package to which access is desired, any attempted tampering will very likely be detected. Although this example of tamper detection is more difficult to circumvent than the preceding example, manufacture of this form of security packaged electronic circuitry requires expensive individual attention to adjustments of the all seems well range for each conductor, in order to optimize tampering detection performance and yet provide for long term reliability, by minimizing effects of aging, and environmental variations, as well as power fluctuations, any of which may cause false alarms.
It is an object of the invention to provide a security package having a housing wall including at least two electrical conductors with a more reliable detection apparatus and method for detecting an occurrence of tampering.